Leadframe alteration to direct compound flow into package

ABSTRACT

A leadframe comprising a downset formed adjacent to an edge of the leadframe so as to direct the molding compound to flow evenly inside the mold cavity. The downset has an upward slope extending from the edge of the frame and levels off with the rest of the frame at a first transition point. The upward slope facilitates the upward flow of the molding compound entering from a bottom gate. Likewise, the leadframe also directs flow in a top gated mold by reversing the orientation of the leadframe or by forming a reverse downset on the leadframe.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 09/898,345, filed Jul. 3, 2001 now U.S. Pat. No. 6,451,629 which isa divisional application of U.S. application Ser. No. 09/489,113, filedJan. 21, 2000 now U.S. Pat. No. 6,278,175.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to packaging of semiconductor integrated circuitsand, more particularly, to a leadframe that directs the molding compoundto flow evenly into the package during the encapsulation process.

2. Description of the Related Art

Semiconductor packaging generally involves incorporating completelyfabricated chips, generally referred to as a die, into protectivepackages so that the dies are protected from environmental contaminantsand handling damage. A common method for packaging integrated circuitscomprises bonding each die to a leadframe and subsequently encapsulatingthe die and a portion of the leadframe in a molded epoxy enclosure.Generally, the leadframe has paddles that receive the die and also haslead fingers that provide easier contact points for external electricalconnection to the electronic components of the die. Hence, the leadframeprovides structural support for the die and allows the encapsulated dieto establish connection with external structures such as printed circuitboards.

More particularly, the leadframe is typically made from a thin sheet ofmetal and the die paddle that is configured to receive the die istypically downwardly recessed to the rest of the frame so as toaccommodate for the thickness of the die mounted to the paddle.Furthermore, the leadframe comprises a plurality of leadframe fingersthat are either stamped or etched on the leadframe and extend from anedge of the die paddle to an edge of the leadframe. An inner end of eachleadframe finger is wire bonded to a bonding pad on the die while anouter end of each finger is designed to form contacts with externalstructures so as to establish a plurality of conductive paths betweenthe die and the external structures. The number and configuration ofleadframe fingers vary depending on the particular die design.

Moreover, an inner section of each leadframe finger and the die itselfare typically encapsulated in a plastic enclosure so that they areprotected from damage and contaminants. During the encapsulationprocess, the die is initially bonded to the leadframe and the die andleadframe structure is placed in a mold cavity wherein the configurationof the cavity defines the shape and size of the resulting protectiveenclosure. Furthermore, a molding compound such as plastic resin isinjected into the cavity so as to complete the formation of a typicallyrectangular shaped enclosure that protects the die and the innersections of the leadframe fingers. Advantageously, the molding operationcan be set up so that multiple dies can be encapsulated in a singleprocess run, thereby providing a cost effective and efficient way ofpackaging integrated circuits.

However, one disadvantage of the standard encapsulation process is thatthe mold cavity configuration and positioning of the leadframe thereinpreclude the molding compound from flowing evenly inside the cavity.This can cause an uneven distribution of resin around the die and theleadframe resulting in a less effective protective structure. Inparticular, the mold cavity typically comprises a top and a bottom platethat mate to form the cavity. The leadframe is placed in the middle ofthe cavity between the two plates. Furthermore, the mold is typicallydesigned so that the molding compound or resin enters the cavity from anopening or gate formed in one corner of the mold. In a bottom gated moldconfiguration, for instance, the molding compound enters the cavity froma bottom corner and the molding compound fills the bottom portion of thecavity more quickly than it fills the top portion of the cavity. Hence,molding compound entering from a bottom gated mold tends to conglomeratenear the bottom of the cavity because its upward flow path is hinderedby numerous horizontally extended leadframe fingers. Likewise, in a topgated mold configuration, the flow of the molding compound also canconglomerate in the top portion of the cavity as the downward flow pathis obstructed by the horizontally extended leadframe fingers.Disadvantageously, non-uniform compound flow rate in the mold and unevenresin distribution between the top and bottom surface of the leadframeare known to cause defects such as voids, pinholes, and knitlines in thecured plastic enclosure.

To address this problem, mold design modifications and leadframealterations have been developed in the past in an effort to achieve amore uniform resin flow inside the mold cavity. For example, U.S. Pat.No. 5,965,078 discloses a mold design used in combination withprepackaged molding compound inserts that are inserted in the cavity ofthe mold with the leadframe and die so as to provide a more uniformcompound flow inside the cavity. However, it can be appreciated thatmold modifications with prepackaged inserts are costly to implement andtherefore undesirable in light of the ever increasing demand for costreduction in semiconductor fabrication.

The prior art also discloses a leadframe having one or more encapsulatediverters wherein the flow diverters are designed to guide portions ofthe upper resin flow to a bottom section of the cavity. (See e.g., U.S.Pat. No. 5,926,695). However, the flow diverter as taught by prior artis applicable only for molds that are top gated wherein the resin isintroduced from a top corner of the mold. Furthermore, the diverter ispositioned away from the gate and therefore unable to direct the flow ofthe compound at the point where the resin first enters the cavity. Infact, the diverter is designed to guide the resin flow only after theresin has already reached the paddle area where air pockets that areknown to cause voids and pinholes are likely to have already developed.Moreover, the leadframe diverters as suggested by prior art areessentially unsupported flaps that can be easily damaged during theencapsulation process. In particular, it can be appreciated that anunsupported thin layer of metal bent at an angle can be deformed orbroken off by the flow of the compound injected into the mold cavity.Furthermore, leadframes with the flow diverters as suggested by priorart are time consuming to manufacture as it requires additionalprocessing steps to ensure that the diverters bend at a particular anglerelative to the leadframe.

Further, the flow diverter disclosed in the patent also requires thatspace on the die paddle be occupied by the opening to permit the resinto flow through. It will be appreciated that given the ever increasingneed for greater density devices, the space on the die paddles and onthe lead frame is becoming increasingly limited. Hence, for many highdensity leadframe designs, forming openings simply for resin flow isimpractical and inefficient.

Hence, from the foregoing, it will be appreciated that there is a needfor a leadframe wherein the leadframe directs the flow of the moldingcompound to flow more evenly inside the mold cavity during theencapsulation process. To this end, there is particular need for analtered leadframe that is able to guide the flow of the compound as thecompound enters the cavity so as to achieve a uniform resin flow ratefrom the outset and thereby minimize defects such as pinholes, voids andknitlines in the molded enclosure. Furthermore, it is desirable that theleadframe is effective in directing compound flow in both top and bottomgated molds. Furthermore, it is also desirable that such leadframealterations can be implemented quickly and cost effectively.

SUMMARY OF THE INVENTION

The aforementioned needs are satisfied by the leadframe of the presentinvention. In one aspect, the present invention discloses a leadframewherein the leadframe comprises a plurality of lead fingers and a diepaddle adapted to receive the die and a second offset positionedadjacent an edge of the leadframe. In this aspect, the second offset ispositioned so as to increase the pressure differential between the firstand the second surfaces of the leadframe so as to increase the flow ofcompound from the first surfaces to the second surface via the spacesbetween the plurality of lead fingers. In this way, the flow of compoundcan be increased without requiring limited space on the leadframe beoccupied by flow openings and the like.

In one embodiment, the offset comprises a downset comprising a downwardindentation formed on a top surface of the leadframe wherein a topsurface of the indentation slopes upwardly towards the paddle until itbecomes level with a general plane defined by the top surface of theleadframe.

In one embodiment, the leadframe further comprises a tie bar wherein anouter end of the tie bar is generally triangular and comprises theoffset. Preferably, in an encapsulation process, the leadframe is placedinside a mold cavity in a manner such that the offset is positionedadjacent a gate. Preferably, the offset is designed to receive a portionof the molding compound entering from the gate and guide the portion toflow toward the second surface of the leadframe. In particular, theoffset produces a localized increase in the pressure of the compoundflowing from the gate. This localized increase in pressure results in agreater pressure differential between the side of the leadframe adjacentthe gate and the side opposite the gate thereby increasing the flow fromthe side adjacent the gate to the side opposite the gate.

Advantageously, the altered leadframe of the preferred embodimentprovides an offset comprising a sloped surface strategically positionedto facilitate the flow of the molding compound as the compound entersthe cavity from a gate. In particular, the altered leadframe splits theflow of the compound as the compound enters from a gate so as to ensurea more even distribution of the compound between the top and bottom ofthe leadframe.

From the foregoing, it will be appreciated that the aspects of thepresent invention provide a new, altered leadframe that is designed todirect molding compound to flow more evenly inside a mold cavity in acost-effective manner. Moreover, the altered leadframe can be adapted todirect compound flow inside mold cavities that are either top or bottomgated. These and other objects and advantages of the present inventionwill become more apparent from the following description taken inconjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top down view of a leadframe of the preferred embodiment;

FIG. 2 is a partial schematic side view of a conventional leadframepositioned inside a mold cavity and the flow direction of the moldingcompound;

FIG. 3 is a partial schematic side view of the leadframe of thepreferred embodiment positioned inside a molding cavity and the flowdirection of the molding compound.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made to the drawings wherein like numerals referto like parts throughout. As will be described hereinbelow, theleadframe of the preferred embodiment provides a leadframe thateffectively directs the molding compound to flow evenly around the dieand leadframe during encapsulation so as to reduce voids and pinholes inthe encapsulated lead frame that typically result from uneven resin flowinside the mold cavity.

FIG. 1 illustrates a general top down view of a leadframe 100 of thepreferred embodiment. As is shown in FIG. 1, the leadframe 100 isgenerally rectangular in shape and is typically made from a thin sheetof metal such as a copper or nickel alloy. As will be described ingreater detail below, the leadframe 100 is designed to providestructural support for a die or chip and also facilitate electricalinterconnection to the components formed on the die. In the typicalpackaging process, the die is encapsulated in a plastic enclosure.Furthermore, the leadframe 100 is also configured to provide a pluralityof conductive paths between the encapsulated die and external structuressuch as printed circuit boards.

As is illustrated in FIG. 1, the leadframe 100 comprises a die paddle102 that is a generally rectangular region located in the center of theleadframe 100 and is formed by a well known stamping or etching process.In particular, the paddle 102 is configured to seat a die 104 anddefines a seating surface 105 that is generally larger than thedimensions of the die 104. Preferably, the paddle 102 is also recessedor downset relative to the rest of the leadframe 100 so as toaccommodate the thickness of the die 104 that is bonded to the paddle102.

Furthermore, as is also shown in FIG. 1, the leadframe 100 comprises aplurality of leadframe fingers 106 that are generally elongated andextend from an edge 110 of the die paddle 102 to an edge 112 of theleadframe 100. Preferably, the leadframe fingers 106 are also formed bystamping or etching the leadframe 100 using a method well known in theart. Furthermore, an inner end 114 of each finger 106 is typically wiredbonded to the die 104, while an outer end 116 of each finger 106 isadapted to connect with external structures such as a printed circuitboard. The leadframe fingers 106 thus effectively serve as a pluralityof conductive paths between the die 104 and external structures.

As FIG. 1 further illustrates, each leadframe finger 106 extendsoutwardly from the edge 110 of the paddle 102 to a dambar 120.Preferably, the dambar 120 has an elongated shape and is formed by thesame etching or stamping process that is used to create the paddle 102and the fingers 106 on the leadframe 100. As is also shown in FIG. 1,each dambar 120 spans in a direction generally perpendicular to theleadframe fingers 106 and is positioned approximately halfway betweenthe edge 110 of the paddle 102 and the edge 112 of the leadframe 100. Inparticular, the dambar 120 is connected to each of the leadframe fingers106 so as to provide rigidity and structural support for the fingers 106during the encapsulation process during which the fingers aresusceptible to damage. Furthermore, the dambars 120 also block andprevent the molding compound from streaking onto an outer section 107 ofthe fingers 106 during encapsulation. Subsequent to encapsulation, thedambars 112 are severed from the fingers 106 in a singulation processthat is well known in the art. In particular, the singulation processseparates the fingers 106 from the dambar 112 so that the fingers 106become disconnected from the dambar 112 and from each other.

As is also shown in FIG. 1, a plurality of tie bars 122 a, 122 b extendfrom the edge 110 of the paddle 102 to the edge 112 of the leadframe100. Preferably, the tie bars 122 a, 122 b are generally elongated andare formed using a well known stamping or etching process. As shown inFIG. 1, the tie bars 122 a, 122 b ensure that the island-like paddleregion 102 located in the center of the leadframe 100 is structurallyconnected to the rest of the frame 100. In particular, an inner end 115a, 115 b of each tie bar 122 a, 122 b is connected to the edge 110 ofthe paddle 102 while an outer end 117 a, 117 b is attached to the edge112 of the leadframe 100.

Furthermore, as shown in FIG. 1, a plurality of dashed lines 126 definea generally rectangular inner region of the leadframe 100 wherein theregion comprises the die paddle 102 and an inner section 130 of theleadframe fingers 106 extending between the paddle 102 and the dambar120. Preferably, once the die 104 is mounted to the paddle 102, theinner region of the leadframe 100 is encapsulated in an enclosure in amanner to be described in greater detail below. Preferably, during theencapsulation process, the inner region of the leadframe 100 is placedinside a mold cavity wherein molding compound is introduced into thecavity from an area adjacent the outer end 117 a of one of the tie bars122 a.

In particular, a plurality of arrows 132 adjacent the outer end 117 a ofthe tie bar 122 a as shown in FIG. 1 indicate the general direction inwhich the molding compound flows into the cavity during theencapsulation process of a preferred embodiment. The arrows 132 show themolding compound entering the cavity from a gate 134 that is locatedadjacent the outer end 117 a of the tie bar 122 a. Preferably, themolding compound flows in a generally horizontal direction from a firstgate, which, in this embodiment, is a bottom lower gate 134 of thecavity and fills the entire cavity so that the top and bottom sides ofthe leadframe are completely covered. Disadvantageously, however, themold configuration and positioning of the leadframe inside the moldcavity often cause the resin to flow unevenly around the leadframe. In abottom gated mold, for instance, the molding compound is shown to favorthe bottom of the cavity because the upward resin flow is often hinderedby the numerous horizontally extended leadframe fingers 106. Likewise,in a top gated mold wherein the compound enters from a top corner of thecavity, the molding compound is shown to gather more around the top ofthe leadframe as the downward resin flow is blocked by the horizontallyextended fingers 106.

In the preferred embodiment, the leadframe 100 is altered so that theleadframe is able to direct the compound flow once the compound entersthe cavity so as to more evenly split the compound between the top andbottom of the leadframe. In particular, the outer end 117 a of the tiebar 122 a positioned adjacent the gate 134 is offset or downset relativeto the horizontally extended leadframe fingers 106 so as to facilitatethe upward flow of the compound. As will be described in greater detailbelow, an indentation 125 is formed on a top surface 123 a of the outerend 117 a of the tie bar 122 a. Preferably, the indentation 125 iscomprises a first and a second sidewall forming a generally v-shapedgroove. Each sidewall slopes upwardly until it becomes level with thegeneral plane defined by the top surface of the leadframe 100.

FIG. 2 provides a partial side view of a conventional prior artleadframe 200 positioned inside a mold cavity 208 during anencapsulation process. As shown in FIG. 2, the conventional leadframe200 comprises a die paddle 202 that is downset and bonded to a die 204.Furthermore, a plurality of leadframe fingers 206 extend outwardly fromthe paddle 202 to an edge 212 of the leadframe 200. As is alsoillustrated in FIG. 2, the die 204 and an inner region 224 of theleadframe 200 are placed inside the mold cavity 208 defined by a top andbottom plate 137 a, 137 b. In particular, the mold cavity 208 defines agenerally rectangular enclosure wherein the inner region 224 of theleadframe 200 is positioned horizontally across approximately the middleof the cavity 208. Furthermore, it is generally known that a moldtypically has an injection port or a gate that is located on either thetop or bottom corner of the cavity that comprises an opening whichallows molding compound to enter the cavity. As is shown in FIG. 2, theillustrated mold 208 has a gate 234 that is formed on the bottom plate137 b, however it is appreciated that the gate can also be positioned onthe top plate 137 a.

As is also shown in FIG. 2, a plurality of arrows 139 demonstrate thepath of the compound flow once the compound enters the mold cavity 208through the bottom gate 234. The arrows 139 illustrate an unevencompound flow wherein more compound is shown to reach a bottom region138 b of the cavity and conglomerate around a bottom surface 201 b ofthe leadframe 200. In particular, the upward flow path of the compoundis obstructed by the horizontally extended leadframe fingers 206,therefore less compound is able to reach the top of the frame 200.Consequently, an uneven flow pattern as illustrated by the arrows 139 isknown to cause defects such as pinholes, voids, and knitlines in theresulting plastic enclosure.

To minimize the occurrence of such defects, the leadframe 100 of thepreferred embodiment is configured to direct resin to flow more evenlyinside the mold cavity so as to minimize the formation of air pocketsduring encapsulation. With reference to FIG. 3, the leadframe 100 of thepreferred embodiment is shown to be positioned inside a conventionalbottom gated mold cavity as described in FIG. 2. As described in detailabove, the leadframe 100 of the preferred embodiment generally comprisesthe paddle 102 bonded to the die 104 and numerous leadframe fingers 106extending outwardly from the paddle 102. Furthermore, as illustrated inFIG. 3, the paddle 102 of the leadframe 100 comprises a first offsetwhich, in this embodiment, is a first downset 105 that is a generallyrectangular indentation formed by a known punch press operation andconfigured to accommodate the thickness of the die 104 that is seated inthe paddle 102.

As is also shown in FIG. 3, the leadframe 100 of the preferredembodiment also has a second offset comprising a second downset 150 thatis a generally v-shaped indentation 152 formed on the top surface 123 aof the outer end 117 a of the first tie bar 122 a. Preferably, theindentation 152 is formed in the same punch press operation as that usedto form the first downset 105. Preferably, the indentation 152 has afirst and a second side wall 159 a, 159 b and each side wall slopesupwardly until it becomes level with a top surface 158 of the leadframefingers 106. Furthermore, the length of the indentation extends from atop surface of the first side wall 159 a to a top surface of the secondsidewall 159 b and the depth extends from a bottom 161 of theindentation to the top surface 158 of the leadframe fingers. As isillustrated in FIGS. 1 and 3, the indentation 152 comprising the seconddownset 150 is located on the end 117 a of the first tie bar 122 a so asto be immediately adjacent the compound gate 134. The indentation 152preferably extends into the flow path 132 of the compound that is beinginjected into the mold 208 during the injection molding process. Thisresults in the pressure of the compound at the injection port 234 beinglocally increased.

Hence, there is a greater pressure differential between the compoundlocated adjacent the bottom surface 171 of the leadframe 100 than theupper surface 173 of the leadframe. As a result of this greater pressuredifferential, more of the compound that is entering adjacent the bottomsurface 171 is induced to flow to the upper surface 173 through theopenings between the lead fingers 106. Hence, as a result of thisgreater flow of compound, the formations of pinholes, knitlines andvoids in the compound is reduced due to the more even distribution ofinjection resin or compound within the mold 208.

Furthermore, it can be appreciated that the above described alteredleadframe can be adapted to direct the flow of compound inside a topgated mold. In particular, the leadframe 100 can be placed in an upsidedown orientation inside a top gated mold so that the second downset 150will guide the compound entering from the top of the cavity to flowdownwardly into the bottom plate. In particular, the leadframe 100 canbe positioned upside down wherein a top surface 101 of the paddle 102faces the bottom plate of the mold. Preferably, when the leadframe 100is placed in an upside down orientation, the second downset 150 willfacilitate the compound entering from the top gate to flow downwardlytoward the bottom of the cavity. Preferably, the leadframe 100 directsthe compound flow so as to more evenly direct the compound between thetop and bottom of the plate and therefore minimize the formation of airpockets that cause defects such as pinholes, voids, and knitlines in theresulting enclosure.

Alternatively, the second offset 150 can also be formed in an upsidedown configuration wherein the indentation is formed on a bottom surface158 of the first tie bar 122 a. In one embodiment, the indentationprotrudes upwardly so as to direct a portion of the compound enteringfrom a top gate to flow downwardly to the bottom of the cavity.Preferably, the configuration of the second offset 150 is identical asthe second downset 150 when the leadframe 100 is positioned upside downinside the mold cavity.

Advantageously, the alteration to the leadframe 100 is not costly toimplement as it can be achieved with relative minor changes to a punchpress die. Therefore, the present invention provides an alteredleadframe wherein the frame allows the molding compound to flow evenlyduring the encapsulate molding process. The alteration is relativelyinexpensive to implement as it involves an extra press operation thatuses existing equipment and tools with minor modifications. Unlike knownleadframe modifications, the altered leadframe effectively distributesthe flow of the compound from the point when the compound first entersthe mold so as to provide a more even flow from the outset.

Although the foregoing description of the preferred embodiment of thepresent invention has shown, described and pointed out the fundamentalnovel features of the invention, it will be understood that variousomissions, substitutions, and changes in the form of the detail of theapparatus as illustrated as well as the uses thereof, may be made bythose skilled in the art, without departing from the spirit of theinvention. Consequently, the scope of the present invention should notbe limited to the foregoing discussions, but should be defined by theappended claims.

1. A leadframe for supporting a die positioned inside a mold cavityhaving an injection port adapted to introduce a molding compound intothe cavity to encapsulate the die, the leadframe comprising: a diepaddle configured to receive the die; a plurality of spaced lead fingerspositioned about the die paddle; at least one tie bar extending betweenthe die paddle and an edge of the leadframe; wherein at least a portionof the tie bar is offset has a downset which extends downwardly relativeto the plane defined by an upper surface of the lead fingers, whereinthe downset is configured to increase the flow of the molding compoundfrom a lower surface of the lead fingers to the upper surface.
 2. Theleadframe of claim 1, wherein the downset is substantially V-shaped. 3.The leadframe of claim 2, wherein the distance between the legs of the Vis about 0.087 inch.
 4. The leadframe of claim 3, wherein the distancebetween the intersection point of the legs of the V and the uppersurface of the leadframe fingers is about 0.009 inch.
 5. The leadframeof claim 1, wherein the leadframe is positioned inside a mold cavity ina manner such that the injection port is located adjacent the offset ofthe tie bar.
 6. The leadframe of claim 5, wherein the leadframe ispositioned inside a mold cavity having a bottom gated injection port. 7.The leadframe of claim 1, wherein the offset is formed using a stampingprocess.
 8. The leadframe of claim 1, wherein the portion of the tie barcomprising the downset is substantially triangular in shape.
 9. Theleadframe of claim 1, wherein a portion of the leadframe comprising thedie paddle is bonded to the die.
 10. The leadframe of claim 1, whereinthe downset is configured to direct the molding compound to flow throughthe spaces between the lead fingers and toward the die.